Douglas Isbell
Headquarters, Washington, DC February 8, 1999
(Phone: 202/358-1753)
Helen Worth
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
(Phone: 240/228-5113)
RELEASE: 99-13
NEAR SPACECRAFT REVEALS MAJOR FEATURES OF EROS
Asteroid 433 Eros is slightly smaller than predicted, with at
least two medium-sized craters, a long surface ridge and a density
comparable to the Earth's crust, according to measurements from
NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft.
NEAR's science instruments observed about two-thirds of Eros
on Dec. 23, 1998, as the spacecraft flew by the asteroid following
an unsuccessful firing of its main engine a few days earlier. A
subsequent successful firing of the engine put NEAR on course to
rendezvous with Eros to begin its planned yearlong orbital mission
starting in mid-February 2000.
Scientists and engineers at The Johns Hopkins University
Applied Physics Laboratory (APL) in Laurel, MD, which manages the
mission, and science team members from affiliated institutions
quickly planned the valuable flyby observations in the wake of the
unsuccessful engine burn on Dec. 20.
During the flyby, 222 photos and supporting spectral
observations of Eros were taken from as close as 2,375 miles
(3,830 kilometers) from the asteroid by the spacecraft's
multispectral imager, infrared spectrometer and radio science
experiment. "The flyby of Eros has given us fundamental
information that will help us plan a better orbital mission at
Eros," said Dr. Andrew F. Cheng, NEAR project scientist at APL.
"It has taken some of the risk out of our orbit insertion maneuver
and early operations."
First observed from the Earth more than 100 years ago, Eros
was known to be an S-type asteroid with high concentrations of
silicate minerals and metal. However, few details about its
structure or composition are observable from the ground. The NEAR
flyby produced evidence of variations in surface color and
reflected light (or albedo) that suggest the asteroid has a
diverse surface makeup. Closer observations during the
comprehensive yearlong orbital study of Eros will be needed to
determine its precise composition.
The science team has determined that Eros is slightly smaller
than originally estimated from ground-based radar observations,
with a size of 21 by 8 by 8 miles (33 by 13 by 13 kilometers),
versus an estimate of 25.3 by 9 by 8 miles (40.5 by 14.5 by 14
km). The asteroid rotates once every 5.27 hours and has no
discernible moons.
The asteroid's density is approximately 1.55 ounces per cubic
inch (2.7 grams per cubic centimeter), close to the average
density of Earth's crust. This makes Eros about twice as dense as
asteroid 253 Mathilde, a C-type, carbon-rich asteroid that NEAR
flew past in June 1997, and about the same density as S-type
asteroid 243 Ida, which NASA's Galileo spacecraft flew past in
1993. Eros and Ida are the only S-type asteroids for which mass
and density have been determined.
Flyby imaging of the asteroid's surface revealed a prominent
elongated ridge that extends along its length for as much as 12
miles (20 km). "This ridge-like feature, combined with the
measurements of high density, suggests that Eros is a homogeneous
body rather than a collection of rubble" such as Mathilde appears
to be, said Dr. Joseph Veverka, of Cornell University, Ithaca, NY,
who heads the mission's imaging team. "It might even be a remnant
of a larger body that was shattered by an impact."
The surface of Eros is pocked with craters. The two largest
craters are four miles and 5.3 miles (8.5 and 6.5 km) in diameter,
less than half the size of asteroid Mathilde's largest craters.
The existence of fewer, smaller craters could be an indication
that Eros has a relatively young surface when compared to Ida.
NEAR and Eros will cross paths again in February 2000. The
spacecraft will then be inserted into orbit around the asteroid
and begin its yearlong study. Images taken during orbit are
expected to have more than 200 times better resolution than those
obtained during the flyby and will be taken from as close as nine
miles (15 km) from the asteroid's surface.
Flyby images of Eros and a related movie, an asteroid shape
model and a chart of spectral observations are available on the
NEAR mission Web site at:
http://near.jhuapl.edu